The imported prosciutto available in the US is either from Parma or San Danielle, and both are excellent examples. But they're a tiny fraction of prosciutto styles in Italy where, until recently, each village made its own, and now at least each region has its own, the flavors influenced by what the pigs eat and the environment in which the hams age. As an American friend living in Italy remarked a few years ago when we were discussing the lack of such hams in the US, "making prosciutto is really easy."
I can't vouch for that, personally, but salt-curing meat isn't exactly a new science. So again, why not quality hams in the US? According to a recent article in the NYT by Harold McGee (author of the excellent book on the science of cooking, On Food and Cooking), there actually is quality ham being produced in the US, and s'prise, s'prise, the key is using mature, fatty pigs instead of rushed-to-market pigs and taking a leisurely approach to the aging. I don't know how one gets ahold of it if you're not a chef using enough to buy a whole leg, but I do know that whenever it is we return home, I've got a few trips to make through ham country.